Gas cluster ion beams (GCIBs) have been used for etching, cleaning, and smoothing surfaces on workpieces, and for assisting the deposition of films from vaporized carbonaceous materials, and for depositing and/or infusing dopants, semiconductor materials, and other materials. For purposes of this discussion, gas clusters are considered nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas clusters may consist of aggregates of from a few molecules to several thousand molecules or more that are loosely bound to form a cluster.
The gas clusters can be ionized by electron bombardment, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges given by the product of q·e (where e is the magnitude of the electronic charge and q is a positive integer having a value of from one to several, representing the charge state of the cluster ion). The larger sized cluster-ions are often the most useful because of their ability to carry substantial energy per cluster-ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently, the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes gas cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage, which is characteristic of conventional ion beam processing.
Presently available cluster-ion sources produce cluster-ions having a wide distribution of sizes, N, up to N of several thousand (where N=the number of molecules in each cluster). In the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, a molecular ion, or a monomer ion. Because of their low mass, molecular ions and/or monomer ions and other very light ions in an accelerated GCIB are often considered undesirable, because when accelerated through an electrical potential difference they acquire much higher velocities than the larger cluster ions.
When used to process a workpiece surface, such high velocity monomer ions tend to penetrate the surface much more deeply than the larger clusters and produce unwanted sub-surface damage, detrimental to the desired process. Accordingly, it has been common practice to incorporate a monomer beam filter in GCIB processing equipment. Such a filter typically uses a magnetic field applied by a (preferably permanent) magnet to the beam to deflect the monomer ions and other low mass ions out of the main GCIB to eliminate their undesired effects on the GCIB process. The monomer and other low mass ions are typically analyzed out of the main GCIB using a downstream aperture that intercepts the deflected light ions, while allowing the heavier ions (which are essentially undeflected) to pass to the workpiece. Commonly-assigned U.S. Pat. No. 6,635,883 to Torti et al. teaches the use of a magnet and aperture for removing monomer and low-mass cluster ions and is incorporated by reference herein in its entirety.
A current measuring device, as for example a Faraday cup, is typically used in GCIB processing equipment to measure the dose of GCIB applied during processing and/or to control the amount of GCIB dose delivered to a workpiece. Such a current measuring device often has an entrance aperture for accepting the beam to be measured. Occasionally, the envelope of a GCIB is ill defined and may tend to fluctuate slightly, so it is useful and desirable to use a beam defining aperture to cleanly define the shape and/or extent of a GCIB prior to current measurement with a Faraday cup. Such a defining aperture assures that the GCIB measured and the GCIB utilized in workpiece processing are the same in extent and that the entire beam used in processing is accepted for measurement by the Faraday cup or other current measuring means, for precise process dosimetry purposes. Commonly-assigned U.S. Pat. No. 6,646,277 to Mack et al. teaches the use of a defining aperture for beam definition prior to workpiece and/or the dosimetry Faraday cup, and is incorporated by reference herein in its entirety.
Many useful surface-processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not limited to, smoothing, etching, film growth/deposition, and infusion of materials into surfaces. In many cases, it is found that in order to achieve industrially practical throughputs in such processes, GCIB currents on the order of hundreds or, perhaps, thousands of microamperes are required to supply the necessary surface processing doses. In general the processing effects tend to increase with increasing GCIB current and/or dose.
Several emerging applications for GCIB processing of workpieces on an industrial scale are in the semiconductor field and in other high technology fields. Due to yield and performance considerations, such applications typically require that processing steps contribute only very low levels of contamination. Although GCIB processing of workpieces is done using a wide variety of gas cluster source gases, many of which are inert gases, in many GCIB processing applications it is desirable to use GCIBs comprising reactive source gases and source gases that can be used to deposit metals, ceramics, semiconductor, and other films, sometimes in combination or mixture with inert or noble gases.
Often halogen-containing gases, oxygen, metals-containing gases, semiconductor-materials-containing gases and other reactive gases or mixtures thereof are incorporated into GCIBs, sometimes in combination or mixture with inert or noble gases. These gases pose a problem for gas cluster ionizer design for semiconductor processing because of their corrosive nature, because they result in etching, sputtering, or deposition of films on impacted surfaces. Often such etching, sputtering, or deposition is part of the intended and desired workpiece processing.
However, apertures such as those used for beam definition and for separating molecular, monomer, and low-mass ions from the processing beam also are irradiated by the GCIB. After extended processing periods involving the processing of many workpieces, the apertures can acquire huge GCIB doses. Such incidental dosing of the apertures can result in formation of contamination of the aperture surfaces due to sputtering, corrosion, and deposition of GCIB components or materials sputtered and/or chemically etched from other surfaces due to GCIB incidence effects. The contaminating materials accumulate on the aperture surfaces, often in the form of poorly-adhered films or accumulations.
Normal thermal cycling, vibrations, or other effects can cause the release of particles of the contaminants from the aperture surfaces. The proximity of such apertures to the workpiece and/or transport of the particles by electrostatic transport effects or other effects can result in very undesirable transport of contaminating particles to the workpiece(s) being processed in the GCIB equipment resulting in spoiled product or low product yields.
With reference to FIGS. 1A and 1B, a conventional beam-defining apparatus 10 for a GCIB processing tool includes an aperture plate 12 and an aperture 14 extending through the aperture plate 12. Aperture plate 12 is supported, held in alignment, electrically grounded and thermally heat sunk by aperture plate support (not shown). Aperture plate 12, which is typically electrically conductive, has a front surface 16 that is struck by a GCIB 20 traveling in the direction of axis 18. The aperture 14 defines the beam and analyzes the beamlet traveling along axis 18, so that monomer, molecular and/or low mass cluster ions are eliminated from the GCIB 20 and only a collimated or filtered portion 19 is transmitted for irradiating and processing a workpiece 22 and for purposes of dosimetry. The aperture 14 has a round cross-sectional profile and is generally disposed within the plane of the aperture plate 12 between the front and rear surfaces 16, 17 of the aperture plate 12.
A portion of the GCIB 20 is intercepted by the front surface 16 of the aperture plate 12 at a roughly annular region 24 surrounding the aperture 14. The angle of incidence is approximately normal to the plane of the front surface 16 of aperture plate 12. After prolonged use, and as a result of sputtering, etching, and/or deposition, contaminants 26 accumulate on the annular region 24 on the front surface 16. Eventually, some of the contaminants 26 may be shed from the front surface 16 in the form of particles that may be transported to the workpiece 22 causing undesirable particulate contamination of workpiece 22. Particles shed from the aperture plate 12 are predominately shed into the GCIB 20 where electrostatic forces and other beam forces facilitate transport to the workpiece 22.
What is needed, therefore, is a beam-defining apparatus for a GCIB processing tool that includes an aperture constructed to reduce the release of contaminant particles of from surfaces near the aperture.